In normal practice a diver carries with him air under pressure contained in a cylinder which communicates with a mouthpiece or mask worn by the diver via a reduction valve and regulator. In the event of failure of the system he must be able to take emergency measures in order to enable him to continue breathing at least for a sufficient time to enable him to rise to the surface of the water. Even if his system has not failed it should be flexible enough to enable him to share it with another diver who has encountered difficulty whilst continuing to use it himself. A further requirement is that the emergency measures should be sufficiently easy to take by a diver who may be in a stressed condition. One of the problems encountered in the design of diving equipment which is capable of performing a variety of functions stems from the fact that it tends to become very bulky and as a result it is liable to interfere with the movements of the diver. In some cases the design of the equipment may result in some limitation in the freedom of the diver to breathe and in other cases his vision might be obstructed by bubbles of exhaled air being discharged in front of his face. The present invention is directed to a breathing system which is both very versatile in its operation and more comfortable to use.